Amplitude controlled torsion rod oscillator for scanning mirror

ABSTRACT

A mechanical oscillator for swinging an optical or other load element back and forth with a substantially constant amplitude. The oscillator is constituted by an erect torsion rod sustained in torsional vibration at its natural frequency by a drive motor energized by drive signals supplied by an external drive generator. The load element is secured to the free end of the rod, and in order to maintain the desired swing amplitude, a fixed pick-up coil is associated with an armature attached to the rod adjacent the free end thereof. Induced in the coil is a control signal whose frequency corresponds to the swing frequency and whose amplitude is proportional to the amplitude thereof. This control signal is applied to a controller adapted to so vary the output of said generator as to maintain a substantially constant swing amplitude.

RELATED APPLICATION

This application is related to our copending application Ser. No. 491,398, filed July 24, 1974 now U.S. Pat. No. 3,921,045.

BACKGROUND OF THE INVENTION

This invention relates generally to torsional oscillators adapted to produce a vibratory motion of high amplitude, and more particularly to a control system for a torsional optical scanner having a relatively broad mechanical bandwidth, said system acting to maintain the scan at a substantially constant amplitude.

Various forms of optical devices are currently in use to chop, scan, sweep or otherwise deflect or modulate a light or other beam of radiant energy. Such optical devices are incorporated in mass spectometers, bolometers, corimeters, horizon sensors and in various instruments which utilize or analyze nuclear, X-ray or laser beams, or beams in the visible, ultraviolet or infra-red region. In recent years, the need has arisen for optical scanners adapted to sweep a light beam across binary-coded bars or similar indicia to produce signal pulses for carrying out various computerized functions.

Existing optical devices for these purposes usually make use of motor-driven discs, drums, mirrors or prisms that are relatively cumbersome and have large power requirements. Also in use is an electromechanically-actuated armature device in which a pivoted armature carrying an optical element is mounted in jewel bearings. Such optical modulators are relatively inefficient and unstable and also lack shock resistance.

The drawbacks characteristic of existing mechanical oscillators are overcome in the resonant torsional oscillator disclosed in the patent to Dostal U.S. Pat. No. (3,609,485), wherein an erect torsional rod, anchored at its base, is electromagnetically-driven at a point adjacent the base to cause an optical element secured to the free end of the rod to swing back and forth at a rate determined by the resonance frequency of the oscillator.

Inasmuch as the ratio of the angular swing at the free end of the rod with respect to the angular swing at the drive point depends on the distance between the drive point and the free end of the rod relative to the shorter distance between the drive point and the base of the rod, the deflection of the optical element at the free end is greatly amplified.

With an arrangement of the Dostal type, certain practical difficulties are experienced when the drive pulses for actuating the rod are derived from an external power source. In the case where the repetition rate of the power source signal is highly stable and corresponds to the natural resonance frequency of the torsion rod, the resultant amplitude of the swing at the free end of the rod is normally high, but is adversely affected by changes in ambient temperature which alter the resonance frequency of the rod.

A torsion rod is a mechanical resonator whose natural frequency is determined by its dimensions and Young's modulus of elasticity. The mechanical bandwidth or response-curve of a torsion rod depends on its Q or quality factor--the higher the Q, the narrower or sharper its bandwidth. That is to say, when a torsion rod oscillator has a high Q, it responds efficiently to drive pulses whose repetition rate exactly corresponds to its natural frequency. However, the response drops sharply as the natural frequency deviates from the frequency of the drive pulses or as the frequency of the drive pulses deviates from the natural frequency of the oscillator.

If, therefore, we have a stable source of drive pulses and the natural frequency of the high-Q mechanical oscillator is caused to depart from its assigned value by reason of temperature changes affecting the torsion rod dimensions, then the amplitude of rod oscillations will markedly decrease. On the other hand, should the source of drive pulses be unregulated, as a consequence of which the repetition rate deviates somewhat from the assigned oscillator frequency, then the amplitude of rod vibrations will decrease sharply to an extent determined by the degree of deviation.

Hence while a highly selective torsion rod oscillator of the type disclosed in the Dostal patent affords increased angular sensitivity, when this oscillator is driven from an external power source whose frequency is subject to variation or when the mechanical oscillator frequency is subject to change as a result of temperature changes, the oscillator in either case does not function efficiently.

SUMMARY OF THE INVENTION

In view of the foregoing, it is the main object of this invention to provide a relatively low-Q torsion rod oscillator having a broad mechanical bandwidth, whereby the frequency selectivity of the oscillator with respect to a drive signal is reduced and the oscillator functions efficiently even when the frequency of the drive signal deviates from the natural frequency of the mechanical oscillator.

Also an object of the invention is to provide a damped torsion rod oscillator which is adapted to swing an optical element or any other element acting as an inertial load over a limited angular range, the angular amplitude of the load accurately following rapid excursions in the amplitude of the drive signal.

A salient feature of the invention is that because of its reduced frequency selectivity, the oscillator may be driven from a non-precision signal source whereby the amplitude of mechanical oscillation is maintained despite deviations of the drive signal frequency from an assigned value. And in those instances where the drive signal source is highly stable but the natural resonance frequency of the oscillator deviates from its assigned value by reason of temperature changes, the amplitude of the mechanical oscillation is nevertheless maintained. Thus the low-Q torsion rod oscillator is capable of reliable operation under circumstances where a high-Q oscillator would fail.

Still another object of the invention is to provide a torsion rod oscillator which is driven from an external power source and which generates a control signal as a function of the amplitude of oscillation, the control signal serving to regulate the power source to maintain a substantially constant oscillation amplitude.

Briefly stated, these objects are attained in a torsional oscillator having an erect torsion rod anchored to a base, an optical or other load element being attached to the free end of the rod and being caused to swing in response to the torsional movement of the rod.

The rod is sustained in torsional vibration by means of a motor energized by a drive signal generator whose frequency substantially corresponds to the natural frequency of the oscillator. In order to stabilize the mechanical amplitude of oscillation, a control signal is derived from the rod as a function of its amplitude, the control signal being applied to the drive signal generator and acting to regulate the amplitude of the drive signal.

OUTLINE OF THE DRAWING

For a better understanding of the invention as well as other objects and further features thereof, reference is made to the following detailed description to be read in conjunction with the accompanying drawing, wherein:

FIG. 1 is a perspective view of one preferred embodiment of a torsional oscillator in accordance with the invention, the oscillator operating as a light scanner;

FIG. 2 is a sectional view of the light scanner;

FIG. 3 is a side view of the light scanner;

FIG. 4 schematically shows the magnetic and electronic circuit arrangement of the oscillator;

FIG. 5 schematically illustrates how the swing of the torsion rod is amplified;

FIG. 6 schematically illustrates how the torsion rod is damped; and

FIG. 7 is an equivalent electrical circuit explanatory of the rod damping action.

DESCRIPTION OF THE INVENTION Oscillator Structure

Referring now to the drawings, and more particularly to FIGS. 1 to 4, there is shown a torsional oscillator in accordance with the invention, the oscillator being adapted to function as an optical chopper to modulate, pulse scan, sweep or otherwise control a beam of radiant energy impinging on the optical element being vibrated by the oscillator.

The oscillator comprises an erect torsion rod 10 whose lower end is anchored in a metal base plate 11 which in turn rests on an insulating mounting plate 12. Secured to the long sides of the base plate are a pair of upright frame elements 13 and 14 made of material such as iron having soft magnetic properties. The ends of these elements are provided with bent-in tabs 13A, 13B, 14A and 14B, which act as pole pieces and define magnetic air gaps. Only one gap (15) is visible in FIG. 1, the other gap 16 being shown in FIG. 3.

Nesting between the frame elements 13 and 14 and lying adjacent the air gaps are a pair of permanent magnet slabs 17 and 18. The ends of these slabs make contact with the frame elements and thereby polarize the tabs in the manner shown in FIG. 4. Thus tabs 13A and 13B are magnetically South and North, respectively, as are tabs 14A and 14B, to provide lines of magnetic flux extending across the gaps 15 and 16.

Mounted transversely on rod 10 and symmetrically positioned thereon is a soft-iron drive armature 19, the opposite ends of the armature being disposed within air gaps 15 and 16. The armature is attached at a point adjacent the base of the rod. Similarly mounted on masthead 10A at the free end of rod 10 is a second and shorter armature 20, this armature being in the same vertical plane as the lower armature.

Mounted above masthead 10A is a reflecting mirror 21 which, when rod 10 undergoes torsional movement, is caused to rotate back and forth about the longitudinal axis of the rod, thereby periodically deflecting a light beam or other beam of radiant energy incident thereto at a rate determined by the resonance characteristics of the oscillator to provide a scanning or chopping action. The nature of the optical element forms no part of the present invention.

Encircling one end of armature 19 is a first stationary drive coil 22 which is cemented or otherwise attached to tabs 13A and 14A, and encircling the other end of the armature 19 is a second drive coil 23 attached to tabs 13B and 14B. Mounted on a backboard 24 near the upper edge thereof is a pickup electromagnet 25. The electromagnet cooperates with one end of the upper armature 20, the electromagnet consisting of a coil surrounding a magnetic core. Thus as armature 20 vibrates, a voltage is induced in electromagnet 25. The drive armature 19 in conjunction with drive coils 22 and 23 constitutes the motor of the oscillator.

OPERATION OF OSCILLATOR

The operation of the oscillator can best be understood in connection with FIG. 4, where it will be seen that drive coils 22 and 23 of the motor are connected to an external power generator 26 which supplies a drive signal whose frequency substantially corresponds to the natural operating frequency of the oscillator.

In the above-identified Dostal patent, the drive generator takes the form of an electronic amplifier whose input is connected to pick-up coil 25 and whose output is connected to drive coils 22 and 23 to form a positive feedback or regenerative circuit producing power at a frequency determined by the mechanical resonance frequency of the torsional oscillator.

Hence should the mechanical resonance frequency of the Dostal oscillator change, the frequency of the power produced by the regenerative circuit changes accordingly and the amplitude of oscillation is maintained.

But in many practical applications, the Dostal feedback arrangement is not appropriate, as, for example, where the torsion rod scanner is associated with a computer system, some of whose functions are controlled by a clock producing pulses at a predetermined repetition rate, the scanner acting to deflect a light beam to read binary-code bars to produce a computer input. In this instance, in order to synchronize the operation of the scanner with the computer it is necessary to drive the scanner externally with a drive signal produced by the generator under the control of the clock. Consequently, the problems mentioned in the background section may arise because of a disparity between the operating frequency of the oscillator and frequency of the drive signal. The drive signal may, in practice, be in sinusoidal, square wave or pulsatory form.

But before considering how these problems are obviated by lowering the Q of the torsional oscillator, we shall first explain how the drive signal actuates the oscillator. In FIG. 4, it will be seen that magnetic pole pieces formed by tabs 13A and 14A and by tabs 13B and 14B are polarized by the permanent magnets 17 and 18 so that tabs 13A and 13B are both "South," whereas tabs 14A and 14B are both "North."

Assuming now that a drive pulse applied to coils 22 and 23 is positive-going, then the motor armature 19 becomes polarized so that the end thereof in air gap 15 is South and the other end in air gap 16 is North. As a consequence, in gap 15 the armature end is attracted to the North side while in gap 16 the opposite armature end is attracted to the South side, thereby imparting a clockwise torque to torsion rod 10 at the armature point. If, on the other hand, the drive pulse is negative-going, the resultant force would be counterclockwise; but in either event, the rod is subjected to a torsional force.

The natural resonance frequency of the oscillator is determined by the moments of motor armature 19 and masthead armature 20 and that of reflector 21 as well as by the physical parameters and Young's modules of the torsion rod 10. To minimize the effect of temperature on rod dimensions, the rod may be fabricated of an alloy such as Ni-Span C or Elinvar, whose elasticity and length are substantially unaffected by changes in temperature within a broad range. However, in some instances because of the excessive stiffness characteristic of many metals having a zero-temperature coefficient, it may be necessary to use such alloys as beryllium-copper which have a greater sensitivity to temperature, in which case the operating frequency of the oscillator will be influenced by changes in ambient temperature.

The torsional movement of the rod at its resonance frequency causes the masthead armature to swing at the same rate and to induce a signal voltage in pick-up coil 25. The frequency of this voltage corresponds to the vibratory rate of the rod and the amplitude thereof is proportional to the amplitude of mirror deflection. In practice, this signal may be produced by a pair of pick-up coils on opposite sides of the masthead armature and connected in series. This signal voltage may be indicated by meter 27, and it may also be used as a control signal to govern the operation of external generator 26 by means of an electronic controller 28.

Controller 28 acts in response to the control signal from pick-up coil 25 to vary the amplitude of the generator output so as to maintain a constant oscillator amplitude. Thus in the controller, the control signal voltage from coil 25 is compared with a set point or reference voltage to produce an error signal which depends on the degree and direction of deviation of the oscillator amplitude from a reference level, the error signal being applied to the generator in a manner tending to null the error signal and thereby maintain the desired amplitude level.

It is not essential that two drive coils be used, and in a low-cost version of the oscillator, one may use a single armature 19, with a single drive coil at one end and a pick-up coil at the other. This is not as efficient as the embodiment disclosed herein, but may be adequate in some applications.

Swing Amplification

In order to obtain a large swing of the mirror, it is essential that the torsional swing at the masthead be large. If the driving force were applied to an armature adjacent the masthead, it would be necessary to have a relatively large air gap to allow for the movement of the armature. But a large air gap would not provide a high magnetic flux density and this arrangement would be inefficient and even impossible.

In the present arrangement, driving armature 19 is adjacent the base of the rod and operates within relatively narrow gaps providing a high flux density. The displacement of the drive armature is relatively small, yet the swing of the masthead is quite large because of the mechanical amplification inherent in the structure.

As shown in FIG. 5, value d represents the short distance between the point of attachment of drive armature 19 and the base, and value D, the long distance between this point and masthead 10A. The angle φ represents the angular swing of the rod at the armature position, and the angle θ, the angular swing of the masthead. Since φ/θ = D/d, it will be evident the larger or greater the difference between d and D, the larger the angle of θ as compared to angle φ. Hence by placing the armature 19 near the base, it becomes possible to drive the rod with a relatively small swing and yet obtain the desired large swing of the mirror on the masthead.

Higher frequencies are obtained by exciting a higher mode in the torsion rod. In this higher mode, a node develops in the middle region of the rod, and the pickup armature operates out of phase with the drive armature. Typical mirror displacements at 10° peak to peak at 2,000 Hz, and 3° at 10,000 Hz.

Damping

As pointed out previously, it is desirable to reduce the Q of the torsional oscillator in order to broaden the mechanical bandwidth of the oscillator and to render it less frequency-selective. The desired reduction in Q is effected by damping torsion rod 10. This damping, as illustrated in FIG. 6, is accomplished by increasing the effective coupling between the tapered section 10X of the rod above the point of armature attachment and the straight section 10Y below this point. This increase in coupling is effected in a manner consistent with the other parameters of the oscillator.

Because of such coupling, some of the energy produced at the free end of the rod to which the mirror or load element is attached is conveyed through point P to the motor associated with the rod where it is damped by the absorbing effect of the magnetic field. The extent of damping is a function of the degree of coupling, the strength of the magnetic field and the geometry of the air gap.

The influence of such coupling on damping can be best understood by considering the torsion rod and its inertial mirror load as a two mesh or loop electrical circuit, as shown in FIG. 7. One mode is the resultant of the load inertia of the rod, as represented by Lm, which is the equivalent inductance, and the spring constant of tapered rod section 10X, as represented by C_(t), the equivalent capacity. The other and higher frequency mode is constituted by the drive armature inertia represented by the equivalent inductance La, the spring constant of the straight rod section 10Y, as represented by capacity C_(st).

As known from Hooke's law, the tension in a torsion rod or any other type of spring is directly proportional to the displacement produced by a force applied thereto (providing the elastic limit of the torsion rod or spring has not been exceeded). The factor of proportionality between displacement and tension is known as the spring constant. The degree of coupling between sections 10X and 10Y of torsion rod 10 is controlled by the ratio of their spring constants. Maximum coupling is attained when the spring constants are the same.

As these spring constants whose electrical analogs are capacitors, approach each other in value, the coupling between the mirror load and drive armature is increased. This increase in coupling for any given rod frequency is accomplished by making the tapered section stiffer; that is, by reducing the slope of this section, and by reducing the diameter of the straight section 10Y. This can be seen from the following relationship: ##EQU1## where w = resultant frequency

w₁ = angular frequency taper section

w₂ = angular frequency straight section

w_(b) = coupling angular frequency.

As the coupling is increased, more energy from the mirror load is absorbed by the motor, thereby lowering the Q of the oscillator and enlarging its mechanical bandwidth. The force developed by the magnetic bias is optimized to afford maximum damping. This is attained by choosing the operating point of the magnetic circuit so that it operates at the maximum energy point of the bias magnets.

While there has been shown and described preferred embodiments in accordance with the invention, it will be appreciated that many changes and modifications may be made therein without, however, departing from the essential spirit thereof. 

We claim:
 1. In combination with a torsion rod oscillator driven by periodic drive signals derived from an external generator and applied to drive means operatively coupled to the rod to cause said rod to oscillate mechanically at a frequency determined by the periodicity of the drive signals, a system for maintaining the mechanical oscillations of the rod at a substantially constant amplitude, said system comprising:A. means responsive to the mechanical oscillations of the rod to produce a control signal whose frequency corresponds to the frequency of the mechanical oscillations and whose amplitude is proportional to the amplitude thereof; and B. electronic controller means responsive to said control signal and coupled to said external generator to vary the output amplitude thereof to an extent maintaining the amplitude of said mechanical oscillations at a substantially constant level.
 2. The combination set forth in claim 1, wherein said torsion rod is provided with an optical element at the free end thereof, the amplitude of whose swing is maintained at a substantially constant level by said system.
 3. The combination as set forth in claim 1, wherein said means to produce a control signal is constituted by an armature attached to said rod adjacent the free end thereof and swinging therewith, and at least one fixed pick-up coil associated with said armature, whereby said control signal is induced therein.
 4. The combination set forth in claim 3, wherein said swinging armature is mounted transversely with respect to said rod, and wherein a fixed coil is associated with each end of the armature.
 5. An optical scanner as set forth in claim 3, wherein said sensor means is constituted by an armature secured to the free end of the rod and a fixed pick-up coil associated with said armature and having said control voltage induced therein.
 6. The combination set forth in claim 1, wherein said controller means is constituted by an electronic controller adapted to compare said control signal with a set point to produce an error signal which depends on the degree and the direction of deviation of the amplitude of the mechanical oscillations from a reference level, said error signal being applied to said generator in a manner tending to null said error signal.
 7. The combination set forth in claim 1, wherein said drive means is constituted by an armature secured to the rod at a position below the free end thereof and an electromagnet associated with the armature and responsive to said drive signal.
 8. The combination set forth in claim 1, further including a mirror secured to the free end of the rod, said mirror swinging to provide an optical scanning action.
 9. An optical scanner comprising:A. a torsion rod oscillator including an erect torsion rod and drive means powered by drive pulses derived from an external generator to produce mechanical oscillations; B. an optical element secured to the free end of the rod and swinging therewith; C. sensor means coupled to said rod to produce a control signal whose frequency corresponds to the frequency of the mechanical oscillations and whose amplitude is proportional to the amplitude thereof; and D. means responsive to said control signal and coupled to said external generator to vary the output amplitude thereof to an extent maintaining the amplitude of said mechanical oscillations at a substantially constant rate. 